Fiber optic multiplex modem

ABSTRACT

A fire alarm network fiber optic multiplex modem includes plural local interfaces, a fiber optic interface, a multiplexor, a fiber optic modem, and a demultiplexor. The multiplexor combines data received at the local interfaces into an outgoing data stream. The fiber optic modem transmits, at a first wavelength, the outgoing data stream to the fiber optic interface and receives, at a second wavelength, an incoming data stream via the fiber optic interface. The demultiplexor separates the incoming data stream into separate data streams, and forwards each of the separate data streams to its corresponding local interface.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a continuation application of commonly-assigned U.S.patent application Ser. No. 12/496,355, which is a continuationapplication of Ser. No. 10/770,249 (now U.S. Pat. No. 7,570,672), thecontents of both of which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

In a typical fire alarm application, copper wires or multiple opticalfibers may be used to convey data. Where fiber is used, one fiber isrequired for each data channel and for each direction; thus a two-way,two-channel system requires at least four fibers. In addition, typicalfire alarm applications also provide for the generation and routing ofone or more analog or digital audio signals, requiring even more wiresor fibers.

SUMMARY OF THE INVENTION

It would be desirable to have multiple channels of data consisting ofdifferent data types and bandwidth requirements convey over a singlefiber optic cable in both directions, using a minimal number ofwavelength channels.

An embodiment of the present invention multiplexes signals in thetime-domain, combining several asynchronous data streams, e.g., networkcommunications and digitized analog audio, into a single data stream.Two wavelengths are utilized; one for each direction, so that the systemcan run full-duplex without regard to the traffic in the oppositedirection.

A fire alarm network fiber optic multiplex modem, according to anembodiment of the present invention, includes one or more localinterfaces, a fiber optic interface, a multiplexor, a fiber optic modem,and a demultiplexor. The multiplexor combines data received at the localinterfaces into an outgoing data stream. The fiber optic modemtransmits, at a first wavelength, the outgoing data stream to the fiberoptic interface and receives, at a second wavelength, an incoming datastream via the fiber optic interface. The demultiplexor separates theincoming data stream into separate data streams, and forwards each ofthe separate data streams to its corresponding local interface. A matingmodem, at the other end of the fiber optic channel, receives at thefirst wavelength and transmits at the second wavelength.

The local interfaces may, in various embodiments, be variouscombinations of, but are not limited to: a network interface, a remoteunit interface, a digital audio interface, and an analog audiointerface.

Where a digital audio interface is employed, in at least one embodimentit is not synchronized with the fiber optic multiplex modem. In such acase, the received digital audio signal may be sampled at pluralintervals within a frame cycle, and a digital audio value placed in theoutgoing data stream based on the samples. The digital audio value maycorrespond with the first sample taken during the frame cycle. A secondvalue can be placed in the outgoing data stream to indicate in which ofthe plural samples the received digital audio signal changed values. Inthis manner, the many samples (say, sixty-four samples) can becompressed to eight bits. Furthermore, a third value may be placed inthe outgoing data stream to indicate whether there has been a change inthe received digital audio signal.

An embodiment of the present invention also includes a fault detectorwhich, upon detection of a fault in a communications path, signals anindication of the fault to a second fiber optic multiplex modem via thefiber optic interface.

In Class A operation, a cross-link to a second fiber optic multiplexmodem normally completes an electrical path in a communications loop.Upon detection a fault in a communications path that extends from thefiber optic interface, the cross-link is disconnected, creating anopen-circuit in the electrical path. Alternatively, if the fault is ashort circuit, a short circuit could be simulated in the electricalpath.

A fault in the network may be detected by, for example, another modem,and the fault information may be transmitted to the present modem viainformation embedded in an incoming data stream received over an opticalfiber directly or indirectly from that fiber optic multiplex modem whichhas detected the fault,

A fault may also be detected responsive to a failure to receive a validincoming data stream via said communications path, or by a failure todetect electrical continuity, for example, in one of the localinterfaces.

An embodiment with analog interface includes an analog-to-digitalconverter (ADC) that converts an outgoing analog signal received at theanalog interface to a digital value. The digital value is thenmultiplexed onto the outgoing data stream. The modem can also include adigital-to-analog converter (DAC) that converts a digital valuedemultiplexed from the incoming data stream to an analog signal at theanalog interface. One embodiment includes both an analog interface and adigital audio interface. Digitized analog audio received over the fiberchannel is converted to analog and transmitted through an analog audioriser, while in parallel, the digitized analog audio is forwarded overthe digital audio interface directly to a matching modem, thus avoidingsignal loss due to excessive conversions between the digital and analogdomain. This technique, called enhanced analog audio, may be used forother analog signals as well.

According to another embodiment of the present invention, a method forcommunicating between nodes in a fire alarm network includes:multiplexing data received from plural local interfaces into an outgoingdata stream; transmitting, at a first wavelength, the outgoing datastream to a fiber optic interface; receiving, at a second wavelength, anincoming data stream via the fiber optic interface; demultiplexing theincoming data stream into separate data streams; and forwarding each ofthe separate data streams to a corresponding local interface.

Alternatively, a fire alarm network fiber optic multiplex modem,according to an embodiment of the present invention, comprises plurallocal interfaces including at least one of, but not limited to, apeer-to-peer protocol control panel communications interface; amaster-to-slave protocol control panel/transponder communicationsinterface; a digital audio interface; an analog audio interface; and afire fighter phone interface. A combiner/decombiner combines datareceived at the local interfaces into an outgoing optical combinedsignal, separates an incoming optical combined signal into itsconstituent data streams, and forwards each of the separate data streamsto a corresponding local interface. The outgoing optical combined signalis transmitted, and the incoming optical combined signal is received,over a single optical fiber through a fiber optic interface.

In one embodiment, the outgoing optical combined signal comprises amultiplexed outgoing data stream at a first wavelength, and the incomingoptical combined signal comprises a multiplexed incoming data stream ata second wavelength. The outgoing and incoming data streams each have adefined fiber frame format.

In another embodiment, the outgoing optical combined signal comprisesplural outgoing optical streams and the incoming optical combined signalcomprises plural incoming streams. Each incoming and outgoing opticalstream comprises data corresponding to a subset (i.e., one or more) oflocal inputs. Each optical stream is assigned to a unique wavelength.

In yet another embodiment, the outgoing optical combined signalcomprises a multiplexed outgoing data stream at an assigned wavelength,and the incoming optical combined signal comprises a multiplexedincoming data stream at the same assigned wavelength, such that at anyinstant at most only one of the incoming and outgoing optical combinedsignals is transmitted over the optical fiber. The outgoing and incomingdata streams each have a defined fiber frame format.

Embodiments of the present invention can work with either or both ofsingle-mode or multimode fiber optic cable.

Class A operation may include, but is not limited to, style 6 and style7 wiring as defined by the National Fire Protection Association (NFPA).Class B operation may include, but is not limited to, style 4 wiring asdefined by the NFPA.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a schematic diagram showing, for exemplary purposes, anillustrative fire alarm network employing an embodiment of the presentinvention.

FIG. 2 is a block diagram of an embodiment of the present inventionfiber optic multiplex modem.

FIG. 3 is a timing diagram illustrating the construction of the fiberframe of an embodiment of the present invention.

FIG. 4 is a schematic diagram illustrating an analog data frameconstructed by an embodiment of the present invention.

FIG. 5 is a block diagram of the field programmable gate array (FPGA) ofFIG. 2.

FIG. 6A is a block diagram graphically depicting the direction controlfunctionality of a network interface with an embodiment of the presentinvention. FIG. 6B provides further detail of the direction controlfunctionality. FIG. 6C is a timing diagram showing an incoming streamreceived and demultiplexed from the fiber. FIG. 6D provides a detail ofthe circled portion of FIG. 6C.

FIG. 7A is a schematic diagram illustrating the use of a presentinvention cross-link in a Class A network configuration to notify theaudio controller of a fiber optic failure or a wiring fault.

FIG. 7B is a schematic diagram illustrating Class B analog audiosupervision using end of line (EOL) resistors in the present inventionmodems.

FIG. 7C is a schematic diagram illustrating the use of a presentinvention cross-link to notify a RUI interface of a fiber optic failure.

FIG. 8A is a schematic diagram illustrating the use of an embodiment ofthe present invention in an internal building or multi-building Class Aconfiguration.

FIG. 8B is a schematic diagram illustrating the configuration of the RUIinterfaces of the system of FIG. 8A.

FIG. 8C is a schematic diagram illustrating the configuration of thenetwork interfaces of the system of FIG. 8A.

FIG. 8D is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 8A.

FIG. 8E is a schematic diagram illustrating the use of the enhancedanalog audio (EAA) feature within the system of FIG. 8A.

FIG. 8F is a schematic diagram illustrating the configuration of thedigital audio interfaces of the system of FIG. 8A

FIG. 9A is a schematic diagram illustrating the use of an embodiment ofthe present invention in an internal building or multi-building Class Bconfiguration.

FIG. 9B is a schematic diagram illustrating the configuration of the RUIinterfaces of the system of FIG. 9A.

FIG. 9C is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 9A.

FIG. 9D is a schematic diagram illustrating the use of the enhancedanalog audio (EAA) feature within the system of FIG. 9A.

FIG. 9E is a schematic diagram illustrating the configuration of thedigital audio interfaces of the system of FIG. 9A.

FIG. 10A is a schematic diagram illustrating the use of an embodiment ofthe present invention within a hub configuration.

FIG. 10B is a schematic diagram illustrating a possible configuration ofthe network interfaces of the system of FIG. 10A.

FIG. 10C is a schematic diagram illustrating an alternativeconfiguration of the network interfaces of the system of FIG. 10A.

FIG. 10D is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 10A.

FIG. 10E is a schematic diagram illustrating the digital audiointerfaces within the system of FIG. 10A.

FIG. 11A is a schematic diagram illustrating the use of an embodiment ofthe present invention within an integrated loop and star configuration.

FIG. 11B is a schematic diagram illustrating the configuration ofnetwork interfaces of the system of FIG. 11A.

FIG. 11C is a schematic diagram illustrating the analog audio interfacesof the system of FIG. 11A.

FIG. 11D is a schematic diagram illustrating the digital audiointerfaces within the system of FIG. 11A.

FIG. 12 is a schematic diagram illustrating methods of fault detection,configuration control and recovery in a simple analog audio Class Aconfiguration using present invention fiber modems.

FIG. 13 is a schematic diagram illustrating methods of fault detection,configuration control and recovery in a simple analog audio Class Bconfiguration using present invention fiber modems.

FIG. 14 is a schematic diagram illustrating methods of fault detection,configuration control and recovery in a simple RUI Class A configurationusing present invention fiber modems.

FIG. 15 is a simplified schematic that covers several differentembodiments of the present invention fire alarm network fiber opticmultiplex modem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fiber Optic Modem andMedia Interface Overview

FIG. 1 is a schematic diagram showing, for exemplary purposes, anillustrative fire alarm network employing an embodiment of the presentinvention. This particular example shows four alarm control panels 4connected in a “master/master” ring network 2 arrangement, wherein thecontrol panels exchange “network communications” data.

As might be found in systems prior to implementation of the presentinvention, some of the master-to-master connections are implemented witha network cable 14, such as a bi-directional differential wire-pairaccording to a standard such as RS-485. Alternatively, pairs of fiberoptic cables have been employed, one for each direction.

In addition, digital or analog audio may be generated by an audiocontroller 10 and circulated throughout all or a portion of the system.Some systems, for example, might provide one or more analog audiosignals each on its own two-wire pair 16. Other systems might provideplural digital audio signals on one pair of wires, using time-divisionmultiplexing techniques.

As shown in this example, for network communications, fire alarm controlpanel 4B uses a network interface card (NIC) 6 that has a left (L) and aright (R) channel. The R channel connected to alarm control panel 4A,and the L channel connected to alarm control panel 4D. (Note that a“card,” as used here and elsewhere throughout this specification,signifies a functional module that may be a separate installable printedcircuit card, or alternatively, a circuit either embedded in a printedcircuit card or some other module.) Of course, data intended for alarmcontrol panel 4B is forwarded internally to a controller (not shown)with the panel 4B.

An audio controller 10 generates and transmits audio content over theaudio cable 16 to other control panels within the network 2, each ofwhich has either its own audio controller, an analog “riser interfacecard” (RIC), or a “digital audio riser interface card” (DARIC). Inexisting systems, both RICs and DARICs recover an audio signal andprovide that signal locally. A RIC simply passes on the analog signal tothe next control panel, while a DARIC regenerates the signal.

Fire alarm control panel 4D similarly has a NIC 6 and audio interface10. The audio signal is also routed, through fire alarm control panel4D's audio controller 10. Here, however, the network and audio signalsare brought to local interfaces of fiber optic multiplex modem 8A, whichtransmits and receives both network data and audio (analog or digital)signals to fire alarm control panel 4C over a single fiber optic cable18A. Note that the audio signal received from control panel 4B on wire16 is received by an audio input option card 11 and routed directly tothe network fiber optic multiplex modem 8A via wire 9.

Fire alarm control panel 4C has a similar arrangement, but has two fiberoptic multiplex modems, the first 8A to communicate with fire alarmcontrol panel 4A over a fiber optic cable 18B and the second 8B tocommunicate with fire alarm control panel 4D over fiber optic cable 18A.Modems 8A and 8B are preferably identical except that the first 8Atransmits over a first wavelength and receives over a second wavelength,while the second modem 8B receives over the first wavelength andtransmits over the second wavelength.

The details of fire alarm control panel 4A are not shown but would becomprise elements similar to those of fire alarm control panel 4Ddiscussed above.

In addition to the network interfaces, each fire alarm control panel maycontrol one or more slave networks 3 having one or more transponders 22.Thus, a fire alarm control panel may have one or more remote unitinterfaces (RUIs) 12, for connecting to transponder units 22 using amaster/slave protocol. Transponders 22 are typically located throughouta building and control and monitor notification appliances 26 such ashorns and strobes, and detection devices 28 such as fire and smokedetectors, using a protocol such as Simplex Time Recorder Co.'s MAPNETII®, IDNET or TrueAlarm®. Although two transponders 22 are shown, itwould be understood by one skilled in the art that each fire alarmcontrol panel 4 may have one or more RUIs, and that each RUI 12 mayinterface with one, two or more transponders 22. A fire alarm controlpanel 4 may also monitor and control notification appliances anddetection devices directly (not shown).

According to the prior art, the RUI would connect directly thetransponders 22 through a RUI network comprising wire or fiber opticcables for each direction (not shown). In addition, audio would berouted on its own cable or fiber to the transponders 22.

Using an embodiment of the present invention, one or more of themulti-cable connections between the fire alarm control panel 4D andtransponder 22A, or between transponders 22, are replaced with a singlefull duplex fiber optic cable. In the example of FIG. 1, all suchconnections have been replaced with fiber optic cable. In fire alarmcontrol panel 4D, this requires the addition of at least one fiber opticmultiplex modem 8C and a mating fiber optic multiplex modem (not shown)in transponder 22A, connected by fiber optic cable 20 which has replacedRUI and audio riser cables. The audio signal captured by the audio inputoption card 11, or generated by the audio controller 10D, is routed tothe fiber optic multiplex modems 8C, 8D via links 13. (The second modem8D is required for Class A operation, discussed below.)

In “Class B” operation, the RUI network would terminate at the lasttransponder 22N in the chain. In “Class A” operation, however, the loopis completed by the addition of a return link 21 from the lasttransponder 22N back to the fire alarm control panel 4D, via fiber opticmultiplex modem 8D within the fire alarm control panel 4D.

The fiber optic multiplex modem of an embodiment of the presentinvention thus converts system audio and communication copper wiring (ormultiple simplex fiber optic cables) to a single full duplex fiber opticlink. In at least one embodiment, the modem is a “pass through” element,and is not addressable. In one design, the fiber optic multiplex modem 8comprises a modem card and a fiber optic media card. The modem cardcontains conversion circuitry, an interface for the fiber optic mediacard, and the various wired media circuitry. The fiber optic media cardcontains the fiber optic components and is assembled as either a “leftport” or a “right port”. These two different assemblies are requiredbecause the media card uses two wavelengths to pass full duplex dataover a single fiber. For example, the left port may utilize a 1310 nmtransmitter and a 1550 nm receiver while the right port utilizes a 1310nm receiver and a 1550 nm transmitter. The fiber optic media card plugsinto the modem assembly to create a left port or a right port assembly.The fiber optic multiplex modem is designed to mount both internallywithin a host equipment box (such as a fire alarm control panel) or in aseparate box. The host equipment may provide power.

Fiber Optic Modem and Media Interfaces

In one embodiment, the fiber optic modem card contains the requiredcircuitry to multiplex and demultiplex the wired media data into asingle data stream for passage through a fiber optic link. A left portmedia assembly on one modem communicates with a right port mediaassembly over the fiber link. The fiber media cards use two wavelengthsto allow full duplex bi-directional communications on a single fiber.

The fiber optic multiplex modem has several local interfaces, includingbut not limited to: a network media interface, a remote unit interface(RUI) media interface, a digital audio riser (DAR) media interface, andan analog audio media interface. In some embodiments, certaincombinations of interfaces may not be permitted or used.

FIG. 2 is a block diagram of an embodiment of the present inventionfiber optic multiplex modem 8, having six logical channels. A generaldescription of each logical channel is listed in Table 1. Spare inputs58 may be available in the modem. In the described embodiment, the sparechannel corresponds with bit 14 of the fiber frame (FIG. 3), which isdiscussed below.

TABLE 1 Modem logical channels Channel Availability DAR always availableAnalog 1 always available Analog 2 always available RUI or Network leftport always available for either RUI or Network left port Network rightport always available Spare always available

The fiber optic multiplex modem described herein includes, in at leastone embodiment, a media converter card that accepts a group of inputs,converts them to digital as required, and multiplexes them onto a singleoptical fiber. The media converter card also receives an incomingoptical data stream, converts it back to an electrical signal, anddemultiplexes it back into its respective components. The modem hasunique discrete (local) interfaces that connect to various I/O sources.The presence and operation of the modem are generally transparent to theconnected equipment and system. All supervision and fault detection ofthe connected wires are the responsibility of the connected equipment,although the modem does monitor wires to convey faults back to theconnected equipment.

A field programmable gate array (FPGA) 59, such as a Xilinx, Inc.Spartan-II series FPGA, provides various functions, includingmultiplexing and demultiplexing functions and interface control. Suchcontrol could also be implemented by a processor or other controlcircuitry. The FPGA transmits the output stream 60 to and receives theinput stream 62 from an optical transmitter and receiver 63 via thedigital/optical interface 61.

From the perspective of the internal logic of the FPGA 59, the fiberinterface appears as two separate paths: transmit and receive. Theintegration (and differentiation) of the two signals is theresponsibility of other components.

Digital Audio Riser (DAR) Interface 51

The digital audio riser (DAR) media interface 51 can be wired for ClassA or Class B DAR communications and contains circuitry that interfacesto a DAR interface card located in a control panel or transponder. Inone embodiment, the DAR channel can be used for any signal of equal orlesser speed than the DAR (768 kbaud). Preferably, direction controlcircuit currently searches for receipt of a framing word within 125 usperiods. If no frame sync occurs within 2 s, the direction controlswitches directions. However, if this channel were used for otherprotocols, a different direction control may be used.

Analog Audio Media Interface 53

The analog audio media interface 53 can provide two channels (or more inalternative embodiments) that can be configured for Class A or Boperation, and contains circuitry to emulate an analog audiocontroller's output and supervision circuit, as well as an analog audioriser interface card.

In the illustrative embodiment, two analog channels are brought into theFPGA 59 via a single interface 56. An analog to digital converter (ADC)54A, such as a Burr-Brown (TI) PCM1801, 16-bit delta sigma dualconverter, digitizes and serializes two channels into one digital stream56A. At the card level, the analog inputs are two separate electricalinterfaces. In the other direction, a digital to analog converter (DAC)54B, such as a Burr-Brown (TI) PCM1725, 16-bit dual channel converter,converts a digital stream 56B into two analog channels. The analogchannels can be used for any signal for which 32 ksps are sufficient,bearing in mind that Mu Law compression is performed on the analog data.The direction control methodology (DC supervision) must be taken intoconsideration if these inputs are assigned to some other input type.

Network/Remote Unit Interface (RUI) 57

The network media interface may be, for example, a “pass through” RS485interface that allows the fiber optic multiplex modem to sit betweennetwork interface cards (NICs) 6 in the host panels 4 (FIG. 1).Communications control is handled by the NICs. Fiber optic multiplexmodems can be configured to provide, but are not limited to, ring, huband star topologies in network systems.

The RUI media interface 12 can be configured for Class A or Bcommunications and may contain circuitry that emulates an RUI card or atransponder interface card's (TIC) RUI circuitry.

These inputs may be re-assigned to an input for which 768 ksps issufficient. This could require that the FPGA direction control timingcircuits change to accommodate speeds other the 9600 and 57.6 k. Forother input types, the direction control circuits would need to be takeninto consideration.

Fiber Frame 70 (FIG. 3)

The FPGA 59 (FIG. 2) operates asynchronously with respect to all inputsexcept the converted analog inputs. Due to this asynchronous nature,compression and regeneration imposes some jitter on the signals. In oneembodiment, the FPGA operates at 49.152 MHz, which is four times themaximum frequency of any input component. The FPGA 59 may internallymultiply the clock by two to minimize the timing error in detecting thefiber frame. Operating at this frequency and constructing a fiber frameas shown in FIG. 3, described below, assures that the jitter is withinacceptable limits.

FIG. 3 shows the construction of the fiber frame 70 of at least oneembodiment of the present invention. The frame is 16 bits in length, andits total duration (1.3 us) is approximately equal to the duration ofone digital audio riser bit. The fiber data may be encoded using, forexample, inverted Manchester encoding, in which each bit has atransition in the center, the first half of the bit time containing theactual data level for that bit. The frequency of the signal on the fiberis 24.576 MHz. The details of each portion of the frame are nowdescribed.

Synchronization

A synchronization “word” preferably comprises the first three bits ofthe frame. These bits are dedicated to maintaining frame synchronizationbetween transmitting and receiving modems. Alternatively, thesynchronization word could be reduced to one bit because there is nodata within the fiber frame that would ever transition every singlefiber frame indefinitely. However, the 3-bit synchronization word isbeneficial because the two low-bit requirement preceding the toggle bitaccelerates the initial frame synchronization detection time.

The synchronization method, termed LLX (low-low-x) or LL-Toggle(low-low-toggle) consists of two consecutive low bits (which actuallyare low-to-high transitions considering both halves of each bit)followed by a “toggle” or “x” bit. The toggle bit changes state in everyconsecutive fiber frame. The toggle bit is the critical portion of theframing word, since no other bit within the data frame (based on datacontent) can toggle in every consecutive frame. A synchronizationdetection circuit within the receiving modem searches for this pattern,and then remembers the previous toggle-bit value once the pattern isdetected. If the pattern ever fails to be repeated in consecutiveframes, then the circuit resumes searching. It is possible that theinitial synchronization word detection could have happened somewhere inthe data portion of the frame. Should this occur, the recurrence of theframing word at the expected time eventually fails (within about 10frames worst case). The search then continues until the pattern is foundagain.

Digital Audio Riser (DAR)

The DAR word comprises bits 3-10 of the fiber frame 70. The DAR operatesat 768 kHz, or 1.3 us per bit. Due to the fact that the DAR operatesasynchronously with respect to the local clock, it is not sufficient totransmit only one bit of DAR data per 10.3 us frame. Since the jitterallowed on the DAR to maintain DAR interface card (DARIC)synchronization is less than 81 ns (in addition to the jitter alreadyimposed by a preceding DARIC, if present), then the fiber frame 70 musteffectively transmit all data received during the sixty-four 49.152 MHzclock cycles of a frame 70.

Jitter may occur due to various factors, including 1) error resultingfrom crystal drift; 2) error resulting from correction in a regeneratedDAR; 3) error added by one 49.152 MHz asynchronous sampling; 4) erroradded by a second 49.152 MHz asynchronous sampling; and 5) a limit tothe amount of error that can be detected and corrected. There can thusbe one-half of a 12.288 MHz clock cycle, or 40 ns margin between theworst-case error stack up and loss-of-sync.

In order to effectively transmit all 64 samples, some compression mustbe done to the data (to allow time for all other signals to betransmitted during a frame). The 64 samples of data can effectively betransmitted in the eight allocated bits as follows:

DAR start level bit: The actual logic level present on the first sample.

DAR transition status: Indicates whether or not a logic transitionoccurred during the frame. For example, high=transition occurred; low=notransition occurred.

DAR transition timing 0-5: Six-bit binary representation of when thetransition occurred (decimal 0-63).

Alternatively, six-bit compression may be accomplished by generating alocal DAR bit timing clock, and using the four bits of transition timingto transmit a correction to that timing. Four bits can provide acorrection of plus or minus (signed) eight decimal clocks. This is threemore than is required: four clock cycles of variation are possibleresulting from a DARIC correction, and an additional clock cycle mayoccur from clock divergence.

Notwithstanding the above discussion, eight-bit compression is currentlypreferred in part because the bandwidth is available, and because thehardware implementation is considerably simpler.

Encoded Analog Channels

Bit 11 of the fiber frame 70 contains data from the analog to digitalconverter 54A (FIG. 2). Since the analog to digital conversion issynchronous with the local clock, a single bit is allocated for thispurpose.

FIG. 4 shows the channel distribution format within the encoded analogdata frame (ADF) 80, which is distributed over many fiber frames 70 (inbit 11 of the fiber frame—see FIG. 3). Because the fiber frame 70 andthe ADF 80 are not necessarily synchronized (with respect to framesynchronization), the ADF 80 begins with a framing word 81 having apattern (in this example, alternating 1s and 0s) that is guaranteed notto appear within the remainder of the ADF. The framing word 81 isfollowed, in this example, by four bits comprising control/status data83, described in Tables 2 and 3 below. The control/status data 83 arefollowed by a “guard” 85 that comprises two high bits. The two-channeldigitized analog audio information then follows.

In FIG. 4, the first number of each slot 87 indicates the analogchannel, and the second indicates which byte of data is contained inthis segment, e.g., “2,3” represents channel 2, byte 3. The data may beMu-Law encoded 16-bit. Each slot 87 is separated by a two-bit guard 85.

Of course, one skilled in the art would recognize that framing word andguard patterns as well as the number of bits in any of the framing word,control/status word, guards or analog channel slots, or the number ofanalog channels, are simply a matter of implementation and could varywithin the scope of the invention.

Network Communication/Remote Unit Interface (RUI)

The network interface may be, for example, an RS485 interface, that actsas a “pass through” device allowing direction control and dataprocessing to be handled by the network interface cards that therespective modem is wired to on each end of the fiber link.

Bits 12 and 13 of the fiber frame 70 (FIG. 3) are dedicated totransmission of network or RUI communication. Network communication maybe, for example, at 57.6 k or 9600 baud, optionally. RUI may operate,for example, at 9600 baud. At 57.6 kHz, or 17.4 us per bit, a singlesample per frame (per 1.3 us) imposes 1.3 us jitter on the digitalstream. This corresponds to 7% of a bit width (1.2% of a bit at 9600bps), which should be acceptable to the receiver. If this jitter is notacceptable, the outgoing data may be reframed. The reframing wouldcreate nominal bit time transitions, delayed by approximately one-half abit time from the input.

Spare and Parity

Bit 14 is available for future expansion. It could be used, for example,for any signal for which a 768 kHz sampling rate would be sufficient,such as a fire fighter phone or other communications. If the signal weresynchronous with the local clock, then the full 768 kHz bandwidth couldbe used. Alternatively the bandwidth could be reduced according to theallowable error on the connected equipment.

Bit 15 contains an even parity bit of the fiber frame 70, excluding thesynchronization word. Parity checking was chosen for simple errordetection on the fiber since a relatively short word (12 bits) istested, and because each input is supervised by its respective connectedequipment.

General Purpose Status & Control

As described above, four bits 83 are available within the analog dataframe (ADF) 80 (FIG. 4) for general-purpose status and control. Thesebits are formatted such that eight bits of data can be communicated.This data may be used to communicate any required status and/or controlinformation between moderns, such as network direction control, resetstatus, etc.

FPGA Design

FIG. 5 is a block diagram of the FPGA 59 of FIG. 2.

The FPGA 59 comprises two largely distinct modules: a receive side and atransmit side. The optical channel, implemented as a wavelength divisionmultiplexed (WDM) channel, is a full-duplex medium. The FPGA 59 gives noconsideration to the timing relationship between the transmit andreceive sides. Internal to the FPGA 59, each of the transmit and receivesides operates constantly. Note that data transmitted on the fiber isone fiber frame (1.3 us) behind the data on the input.

The terms “TX” and “RX” throughout most of this description are withrespect to the optical interface, while the terms “IN” and “OUT” arewith respect to the wired interface. The terms “input stream” and“output stream” are with respect to the optical interface.

Digital Audio Riser (DAR) Interface Module 101

The DAR interface module 101 receives an asynchronous digital audiosignal from the external DAR interface 51 (FIG. 2) and compresses thesignal to an 8-bit parallel value as described above. The DAR interfacemodule 101 then provides its output to the multiplexer/transmittermodule 109.

The DAR interface module 101 also receives data in the same format fromthe demultiplexer/receiver module 111, decompresses the data, andtransmits it serially back out of the device. The compression operationcompresses sixty-four 49.152 MHz samples as follows (refer to FIG. 3):

Frame bit 3: Start Level. This bit is the actual logical state of thefirst sample of the DAR.

Frame bit 4: Transition Status. This bit indicates whether or not alogic transition took place during the frame. A transition may not occurfor two reasons: either there were actually two subsequent bits on theDAR at the same level, or the clock source to local clock difference andtiming coincides such that one DAR bits transition falls on either sideof a sample period.

Frame bits 5-10: Transition Timing 0-5. These six bits indicate when atransition took place.

In the event that an edge occurs ‘prematurely’ (two transitions within asample period), then the second edge is only extended by one clockcycle. In the event of a corrected bit width resulting from a DARICcorrection (which would be sixty local clocks instead of sixty-four),then the correction is redistributed over two DAR bits instead of theone bit that the correction was contained in previously. This should bewell within the correction window limits of a subsequent DARIC.

In the unlikely event that the correction distribution discussed aboveis a problem, there may be several possible ways to rectify it. Forexample, the spare bit (bit 14 in FIG. 3) in the fiber frame 70 can beused to transmit a “dual-transition” bit. If this bit were set, then thereceiver would automatically output a transition sixty clocks after thefirst transition whose timing was conveyed in the timing word.Alternatively, since there are only sixty-three possible transitiontiming values (since the first sample is actually transmitteduncompressed), the 64^(th) value could be used to indicate that a“dual-transition” occurred, and the same action as in alternative ‘1’can be taken.

Analog Audio Interface Modules 103, 105

The analog audio interface modules 103, 105 are responsible forcontrolling the ADC 54A (FIG. 2) and the DAC 54B (FIG. 2). Bothconverters 54A, 54B require the same control signals. For the ADC 54A,these signals may be generated through division of the local clock. Forthe DAC 54B, these signals may be generated locally, but reset eachframe by the received framing word. An off-chip phase-locked-loop (notshown), such as a 74HCT4046A, can be used to assure that the DAC 54Breceives the appropriate number of system clocks per left-right clock.The DAC interface 105 receives incoming (i.e., from the optic fiber)analog data frames 80 (FIG. 4) from the demultiplexer/receiver module111. The DAC interface 105 decodes the frame, and provides the dataserially to the DAC 54B (FIG. 2). The analog data may be, for example,16-bit, Mu Law compressed to 8-bit for transmission over the fiber.

As previously discussed, the analog audio interface modules 103, 105 mayalso be responsible for communication of status and control information.Four bits available within the ADF 80 may be formatted to contain eightbits of status and control information, for example, as shown in Table 2below. The bits may be updated, for example, at a rate of once every 500us. Any status change reaches the mating modem within 500 us. Table 3shows the definitions applied to the various status and control bits(STAT_CNTLx) of Table 2.

TABLE 2 ADF control bit selects ADF Control Bit assignments Group SelectData 3 2 1 0 0 0 STAT_CNTL2 STAT_CNTL1 0 1 STAT_CNTL4 STAT_CNTL3 1 0STAT_CNTL6 STAT_CNTL5 1 1 STAT_CNTL8 STAT_CNTL7

TABLE 3 Status and Control bit assignments STAT_CNTL number AssignmentDetails 1 Reset flag Reset signal to/from mating modem. Corresponds toreset at the remote reset input. 2 Analog ch1 Riser channel statuschange flag. flag 0 = normal; 1 = fault 3 Analog ch2 Riser channelstatus change flag. flag 0 = normal; 1 = fault 4 RUI flag RUI channelstatus change flag or attempt reset flag (depending on modemconfiguration). 0 = no change; 1 = change 5 Fiber fault Signal to matingmodem indicating LED flag a fiber fault. It is possible that one modemis capable of receiving, but not the other. This assures that bothmodems indicate the fault if there is one. 6 Fiber trouble Signal tomating modem indicating flag a fiber trouble. It is possible that onemodem is capable of re- ceiving, but not the other. This assures thatboth modems enter degraded mode if either modem cannot receive. 7 Notused. Available for future expansion. 8 Not used. Available for futureexpansion.

The analog audio interface also has the capability to operate in an“Enhanced Analog Audio” (EAA) mode. Standard analog audio imposes asix-modem pair limit due to distortion and noise increases each time theanalog audio is converted to and from digital. EAA imposes its ownlimitations, but eliminates the six-modem pair limit. This can beaccomplished by routing the received analog data frame 80 to the DARwiring interface 51 (FIG. 2), thus passing the ADF directly to the nextmodem, in parallel with the analog riser. The result is that thereceiving modem does not need to re-digitize the audio signal from theanalog audio riser, and no distortion or noise is added. Downstreammodems may be configured to enable or disable EAA, and to select whetheror not the modem has the DAR wired as the EAA input.

Network/RUI Interface Module 107

The networldRUI interface module 107 (FIG. 5) is the simplest module,since the speed of the fiber frame is dramatically higher than the speedof either the network communications or RUI, and it is the leastsensitive to jitter of all interfaces. This module samples the inputonce per fiber frame, and makes the sampled data available to themultiplexer/transmitter module 109. This module also receives parallelnetwork/RUI data back from the receiver/demultiplexer module 111 andtransmits it serially out the local side. The network/RUT module 107 isalso responsible for direction control of the RS485 transceivers.

Multiplexer/Transmitter 109

The multiplexer/transmitter module 109 receives parallel data from eachof the interface modules 101-107 once every fiber frame. It generates asynchronization word, and inserts all data into the fiber frame 70depicted in FIG. 3. The multiplexer/transmitter module 109 is thecontrolling module for the synchronous sampling and packaging of allinput data. It indicates to each of the input modules when astart-of-frame has occurred, so that each module knows when to prepare anew set of samples.

Demultiplexer/Receiver 111

The demultiplexer/receiver 111 receives the fiber frame 70 from theoptical interface. It continuously samples the input in search of asynchronization word. Upon detecting a synchronization word, the module111 samples each bit of the frame at the proper interval to recover thedata. It then feeds this data in a parallel format back to each of thelocal interface modules 101-107, also sending a start-of-frame pulse toindicate to each of the modules the proper time to update respectiveoutput.

Routing/Direction Control

The FPGA 59 must control the off-chip direction controls to theinterface transceivers on the “local” side of the card. The off-chipdirection control can be handled as follows:

DAR Interface: A two-second counter (not shown) in the FPGA 59 generatesa detection time-out. The FPGA monitors for the presence of an activeDAR on the input, and switches the direction control output if no DAR isdetected after the expiration of the time-out. This function continuesto toggle the direction control output every two seconds until an activeDAR is detected.

Analog Interface: Analog routing control is accomplished throughdetection of an end-of-line resistor (EOLR), using logic to produce theappropriate reaction based on each module's configuration. Depending onthe configuration of a particular modem, as well as its current status,it will change its state contingent upon either the failure to detectthe EOLR or a flag sent by its mate. Note that, in one embodiment of thepresent invention, the EOLR is present on the receiving modem for classB applications, and is present on the transmitting modem for class A.

RUI Interface: RUI routing control is accomplished by an algorithmimplemented in logic that recognizes electrical changes on the wiringand associates such changes with higher-level state-changes. Since thereare no data direction control requirements for RUI, the FPGA 59 pays noattention to actual traffic flow on this interface. Voltage mode data onthe input and current mode responses are passed through any time theyare received. Continuity of the wiring is supervised via the DCcomponent of the communications.

Network Interface: FIG. 6A is a block diagram graphically depicting thedirection control functionality of the network interface. A “bridge”configuration is shown for simplicity. Two RS-485 transceivers 61interface with the left and right interfaces of a network interface card6. The FPGA 59 monitors for a negative going edge (start bit) on theoptical side input of each network channel. When a transition isdetected, the direction control switches such that the data istransmitted out the local side. Once one character period (based onnetwork speed and protocol) has passed, the direction control reverts toreceive mode. The actual transmit enable pulse is one half of one bittime shorter than the entire character length. The result of this timingis that the direction control reverts to receive mode half way throughthe stop bit, which is a logic high. As a result, the network datashould be unaffected, and this half bit advance provides ample time forthe monitor circuit to resume searching for the next negative goingedge. Both the optical transmit and receive are always enabled, sincethe two directions are on two separate wavelengths and cannot interferewith one another.

FIGS. 6B-6D illustrate this concept at a slightly lower level. FIG. 6Bagain shows a modem 8 with, for simplicity, just one of the network/RUIRS485 transceivers 61. The transceiver 61 has a driver 153 fortransmitting data and a receiver 151 for receiving data. Data istransmitted and received on a single differential pair of wires 156. Tocontrol the direction, the FPGA 59 controls a transmit enable signal155, which is normally not asserted, such that the transceiver 61defaults to a receive mode.

As seen in the timing diagram of FIG. 6C, an incoming stream 158received and demultiplexed from the fiber is normally at some level, say5 VDC, when inactive, i.e., at rest. At the same time, the transmitenable signal 155 is normally low so that the receiver 151 is enabled toreceive network or RUI communications, and the transmitter 153 disabled.

When a start bit 153 is detected in the incoming stream 158, the FPGA 59asserts the transmit enable signal 155 at 154, while passing thecommunications data to the driver 153, which begins transmitting thedata immediately. The transmit enable signal 155 remains asserted for afixed period, covering the start bit 153, the data 155 (which in oneembodiment may be eight or nine bits), and half of the stop bit 157.

FIG. 6D is a detail of the circled portion of FIG. 6C, illustrating thatthe transmit enable signal 155 is de-asserted at 161, halfway throughthe stop bit 157.

Error Detection and Trouble Reporting

The primary responsibility of error detection and reporting lies withthe connected equipment. In the event of a card failure, the connectedequipment may perceive the following errors:

DAR: DAR interface cards (not shown) within control panels ortransponders supervise receipt of the DAR signal, and pass a troubleindication to the connected transponder interface card (TIC) in theevent of a DAR failure.

Analog Riser: Any connected analog risers are monitored by the analogaudio controller and the riser interface card (RIC), and any amplifiersvia monitoring of the supervision tone, and monitoring of theend-of-line resistors.

Class A Analog Audio

Class A analog audio operation forms a loop beginning with an audiocontroller in a fire alarm control panel. Audio is sent from the audiocontroller class B output, i.e., the “primary,” to the varioustransponders on a loop, and wired for verification at the analog audiocontroller class A return. This way, the audio controller can determinewhether there is a break or fault in the loop. However, when the presentinvention modem is put into place, optical fibers replace one or morelinks in the loop, and there is no longer a complete electrical pathfrom the audio controller's class B output to its class A return. Normalverification of the path cannot be done under these circumstances.

FIG. 7A is a schematic diagram illustrating the use of a cross-link(x-link) 313 to notify the audio controller 312 of a fiber opticfailure, or a wiring fault on the opposite side of the optical isolationdescribed above. The various modems 300-305 within the loop are able todetect a fault in the loop and can communicate this fault information tothe tail end modem 305 using ADF control bits as described above, andbetween wire-connected modems (e.g. 303, 304) by removing an end-of-lineresistor (EOLR), e.g., 318.

As shown in FIG. 7A, analog audio is transmitted from the audiocontroller 312 “Class B” output to the head end fiber optic multiplexmodem 300, which multiplexes the data with other data, e.g., RUI (notshown), and transmits the multiplexed output stream to the nexttransponder in the loop via optic fiber 314. As far as the audiocontroller 312 is concerned, the loop appears normal because thecross-link 313 between the two modems 300, 305 completes the electricalloop. When a problem is detected, the problem information is sent to thetail end modem 305. However, the fiber optic multiplex modem of thepresent invention is “transparent,” i.e., it cannot communicate thisinformation directly to the audio controller. Instead, modem 305 opensswitches 315, thus breaking the electrical loop. The audio controller312 interprets this open loop as a fault and begins transmitting audioout both the class B output and the class A return.

As shown in FIG. 7B, Class B analog audio supervision is accomplished bymonitoring an EOLR. A prior art system would typically have a singleEOLR at the end of the analog riser (across the wires). Where presentinvention fiber optic multiplex modems are installed, the final EOLR 319is not electrically connected across the system. Therefore, certainmodems 306, 308, 310 themselves present an EOLR 316 across the wires,and supervise downstream for the same. Referring to FIG. 7B, modem 306presents EOLR 316, which the audio controller 317 monitors. Modem 307monitors for the EOLR, which is presented by modem 308, and so forth. Ifany modem fails to detect an EOLR, then it conveys that information toits mate via the status and control packet within the ADF as describedabove. The modem receiving this information then opens its EOLR switch,e.g., 316, to covey the fault back upstream. When modem 306 opens itsEOLR 316, the audio controller 317 no longer detects the EOLR, andreports the trouble at the user interface.

Network: Network communications is inherently supervised by thetransmitted intelligence; in the event that the network fails, theconnected equipment is unable to respond to polls, and therefore anetwork trouble is reported.

RUI: Since RUI is a master/slave protocol, a failure of the fiber isperceived as a failure of all connected devices, and hence the RUIchannel itself.

Class A RUI operation forms a loop beginning with the RUI interface in afire alarm control panel. RUI data is sent, from the RUI interface classB output, around the loop to the various transponders on the loop, andreceived for verification at the RUI interface class A return. This way,the RUI interface can determine whether there is a break or fault in theloop. However, when the present invention modem is put into place,optical fibers replace one or more links in the loop, and there is nolonger a complete electrical path from the RUI interface's class Boutput to its class A return. Verification of the path cannot be doneunder these circumstances.

FIG. 7C is a schematic diagram illustrating the use of a cross-link(“x-link”) 90 to notify the RUI interface 12 of a fiber optic failure.The reference numbers are intended to correspond with those of FIG. 1for exemplary purposes. The various modems within the loop (includingthose shown in FIG. 7C, as well as those not shown, for example, withinthe transponders 22 of FIG. 1) are able to detect a fault in the loopand can communicate this fault information to the tail end modem 8Busing ADF control bits as described above.

As shown in FIG. 7C, RUI data is transmitted from the RUI interface 12“Class B” output to the head end fiber optic multiplex modem 8A, whichmultiplexes the data with other data, e.g., audio data (not shown) andtransmits the multiplexed output stream to the next transponder on theloop via optic fiber 20. However, as far as the RUI interface 12 isconcerned, the loop appears normal because the cross-link 90 between thetwo modems 8A, 8B completes the electrical loop. When a problem isdetected, the problem information is sent to the tail end modem 8B.However, the fiber optic multiplex modem of the present invention is“transparent,” i.e., it cannot communicate this information directly tothe RUI interface. Instead, modem 8B opens switches (or relays) 92, thusbreaking the electrical loop. The RUI interface 12 interprets this openloop as a fault and begins transmitting RUI data out both the class Boutput and the class A return.

In addition to the supervision that the connected equipment provides, anembodiment of the fiber optic modem may provide LED indicators for eachof the interfaces. Although not capable of detecting all troubles, themodem can use these LEDs to indicate some trouble conditions, such as afiber fault or degraded mode operation on RUI or analog channels.

A second trouble indicating method may be used. Generally, an opencircuit on a class A loop may be generated on the modem's output in theevent of a fiber communication failure so that a degraded mode ofoperation is entered by the receiving circuit. This function may berequired so that a fault on the source side of the fiber invokes theappropriate response on the output side. Alternatively, a modem may alsogenerate a short circuit on the local side, to duplicate the actualnature of the fault.

An embodiment of the present invention modem has two types of fiberfault conditions: “fiber fault” and “fiber trouble”. A fiber faultcondition only effects the fiber fault LED, while a fiber troublecondition invokes a degraded mode operation, if applicable to aparticular interface. Fiber faults may occur without fiber troubles, butfiber troubles will never occur without fiber faults (LED indication). Afiber fault condition may be triggered, for example, by momentary lossof synchronization or a single parity error. The fiber fault LED lightsfor ½ second in the event of any problem with the optical data path. Thefiber trouble condition is determined by continued parity errors orfailure to synchronize for an extended period (e.g., 125 us). In theevent of a fiber trouble condition, analog audio and RUI enter adegraded mode operation or report wiring faults, depending on style ofoperation, and troubles might be latched by the panel depending on theirnature.

To allow operation with fiber troubles (but not fiber faults), filteringcan be applied to the flags that are exchanged between moderns. Thefiltering prevents erroneously receiving a flag while corrupt data isbeing received, and increases the likelihood that a flag sent duringimperfect operation will be received. A flag missed due to corrupt datamay cause a delay equivalent to the duration of the fault condition forall interfaces except RUI, where it could cause a worst case delay of 14s (the modem's automatic recovery time of a class A RUI loop). Testshave indicated that fiber faults do not occur over single mode fiberwith 20 dB of attenuation, but do occur once every few days overmultimode fiber of 5000 feet plus an air gap attenuator to total 6 dB.Fiber troubles have not occurred with 20 dB single mode or 15,000 feetand12 dB of multimode.

Definitions

ADF: Analog Data Frame. An embodiment of the Fiber Optic Modem encodestwo channels of 32 ksps audio data in a format called the analog dataframe. Although a particular format is described above, it would beobvious to one skilled in the art that other formats may be used aswell.

DAR: Digital Audio Riser. The DAR media interface is an RS485 interfacethat acts as a “pass through” device that operates the same as a DARICinterface.

DARIC: Digital Audio Riser Interface Card. The interface card that isnormally used within a control panel or a transponder as a connectionpoint and receiver/regenerator for the Digital Audio Riser.

EAA: Enhanced Analog Audio. This refers to the feature that allows theADF to be transmitted digitally on the DAR channel in parallel with theanalog channel (etc.), thereby allowing a digitized analog riser toreach all modems with this feature enabled without being converted toanalog and back in between.

RUI: Remote Unit Interface—RUI interface transmits data to transpondersas a voltage level and receives data back as current.

TIC: Transponder Interface Card—The TIC resides in a transponder andtransmits data to the RUI interface or RUI card as current and receivesdata as voltage. The TIC interface also monitors the integrity of thewires when required.

Example Configurations

FIG. 8A is a schematic diagram illustrating the use of an embodiment ofthe present invention in an internal building or multi-building Class Aconfiguration. FIGS. 8B-8F show sample configurations for variousinterfaces for the system of FIG. 8A. The system shown could represent,for example, four control panels within a single building, oralternatively, four buildings each with one panel. Of course, othercombinations may be configured with varying numbers of panels perbuilding.

FIG. 8B is a schematic diagram illustrating the configuration of the RUIinterfaces of the system of FIG. 8A.

FIG. 8C is a schematic diagram illustrating the configuration of thenetwork interfaces of the system of FIG. 8A. Note that at least oneembodiment of the modem does not support the concurrent use of both theRUI and network interfaces.

FIG. 8D is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 8A.

FIG. 8E is a schematic diagram illustrating the use of the enhancedanalog audio (EAA) feature within the system of FIG. 8A.

FIG. 8F is a schematic diagram illustrating the configuration of thedigital audio interfaces of the system of FIG. 8A

FIG. 9A is a schematic diagram illustrating the use of an embodiment ofthe present invention in an internal building or multi-building Class Bconfiguration.

FIG. 9B is a schematic diagram illustrating the configuration of the RUIinterfaces of the system of FIG. 9A.

FIG. 9C is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 9A.

FIG. 9D is a schematic diagram illustrating the use of the enhancedanalog audio (EAA) feature within the system of FIG. 9A.

FIG. 9E is a schematic diagram illustrating the configuration of thedigital audio interfaces of the system of FIG. 9A.

FIG. 10A is a schematic diagram illustrating the use of an embodiment ofthe present invention within a hub configuration. A hub configurationconsists of a main loop with nodes connected in a radial manner. In theillustrative configuration shown, modems connect the hub node to theremote nodes. Where T-tapping is not allowed, then the optional fibershown is needed if an audio interface is used.

FIG. 10B is a schematic diagram illustrating a possible configuration ofthe network interfaces of the system of FIG. 10A.

FIG. 10C is a schematic diagram illustrating an alternativeconfiguration of the network interfaces of the system of FIG. 10A.

FIG. 10D is a schematic diagram illustrating the configuration of theanalog audio interfaces of the system of FIG. 10A. In this diagram, thehead-end audio cabinet is not shown. If Class A is required, head andtail-end modems require x-link connections.

FIG. 10E is a schematic diagram illustrating the digital audiointerfaces within the system of FIG. 10A.

FIG. 11A is a schematic diagram illustrating the use of an embodiment ofthe present invention within an interconnected loop and hubconfiguration. In this configuration, the fiber modem connects the twoClass A network loops in tandem. Modems also connect several remotenodes to the loop.

FIG. 11B is a schematic diagram illustrating the configuration ofnetwork interfaces of the system of FIG. 11A.

FIG. 11C is a schematic diagram illustrating the analog audio interfacesof the system of FIG. 11A.

FIG. 11D is a schematic diagram illustrating the digital audiointerfaces within the system of FIG. 11A.

Analog Channel Routing Control

FIGS. 12-14 and Tables 4-8 show the methods of fault detection,configuration control, and recovery. Each figure shows a sampleconfiguration, with individual faults indicated by small labeled boxes.The tables show the method of detection of the fault, and the process ofentering degraded operation. The tables also show the method of recoveryfrom degraded mode.

FIG. 12 illustrates a simple analog audio Class A configuration with onemaster control panel 204 and three transponders 222. One of thetransponders 222C is on the “local” side of the fiber optic links,although it could be located over 1,000 feet from the control panel 204.The small labeled boxes represent potential optical fiber breaks orfaults and correspond with Table 4 as described below.

The master control panel 204 contains an analog audio controller 210which has primary and secondary ports. In “normal” operation, thiscontroller 210 expects to transmit the audio signal from the primaryport and to receive the signal at the secondary port for verification ofthe circuit. Modem A 208A receives the analog signal, and passes it,through its closed connection x-link 292A to Modem F 208F, while alsodigitizing the signal and multiplexing it with network data and anycontrol/status data for transmission by fiber to transponder 222A. Fromthe viewpoint of the analog audio controller 210, Modem A 208A lookslike a riser interface card (RIC). In fact, Modem A 208A is configured(for example, by setting DIP switches) to default to RIC behavior.

The analog signal passes through Modem F's (208F) closed connectionx-link 292B to RIC 207C, which passes the analog signal through ananalog audio riser (not shown) and finally back to the secondary port ofthe analog audio controller 210.

Within transponder 222A, Modem B (208B), receives and demultiplexes themultiplexed signal, extracting the analog audio signal and forwarding itto the primary port of RIC 207A. Modem B 208B is configured to defaultto appear to RIC 207A as an analog audio controller (AAC). In this AACmode, Modem B 208B presents an end-of-line (EOL) resistor 215B orequivalent to RIC 207A.

RIC 207A passes the audio analog signal out through its secondary portto Modem C 208C, which multiplexes the signal with communicationssignals and transmits the multiplexed data stream to Modem D 208D overfiber CD. Modem C 208C is configured to default to “RIC” mode.

Transponder 222B has components 208D, 208E, 207B that parallel those oftransponder 222A. Transponder 222C, on the local side of the fiber opticlinks, that is, connected electrically with the control panel 204, hasthe “tail end” modem, Modem F (208F) and a RIC 207C.

Table 4 below, shows how the fiber multiplex modems 208 of FIG. 12 reactwhen a fault, i.e., a short or open circuit, is detected at each of theidentified boxes. In Table 4, action begins with the left-most entry ina row, designated with an asterisk “*”. For each modem, the triggeringevent is identified first, then (separated from the event by a colon“:”) the action taken by the modem.

For example, if a fault occurs at Box 2, corresponding to Fault 2 inTable 4, Modem C 208C cannot sense Modem B's end-of-line (EOL) resistor215B. Modem C, which defaults to RIC mode, then switches AAC mode, andsends a flag to its mate, Modem D. Modem D receives this flag (thetrigger) and opens its EOL resistor (the action). Modem E, upondetecting the loss of the EOL resistor 215B, switches to AAC mode andsends a flag to its mate, Modem F. Modem F, upon receiving the flag,opens the x-link switch 292B. The analog audio controller 210 detectsthe open circuit created by the opening of switch 292B and takescorrective action.

The remaining rows of Table 4 are to be interpreted similarly for thedifferent breaks or faults as indicated by the small corresponding boxesof FIG. 12.

TABLE 4 Class A analog audio fault detection and recovery ConfigurationModem A Modem D Modem F head end Modem B Modem C generic Modem E tailend Fault RIC generic AAC generic RIC AAC generic RIC RIC 1 No detectionor recover; controller handles faults. 2 *no EOL: flag: mode no EOL:flag: mode AAC & RIC & open mode AAC & open xlink set flag EOL set flag2 restore *EOL: mode flag: mode EOL: mode flag: RIC & clear AAC & RIC &clear close xlink flag close EOL flag 3 *no EOL: flag: mode AAC & openxlink set flag 3 restore *EOL: mode flag: RIC & clear close xlink flag 4No detection or recover; controller handles faults. 5 No detection orrecover; controller handles faults. AB *no fiber: no EOL: flag: no EOL:flag: open EOL mode AAC & mode RIC & mode AAC & open xlink set flag openEOL set flag AB restore *fiber: EOL: flag: EOL: flag: close EOL mode RIC& mode AAC & mode RIC & close xlink clear flag close EOL clear flag CDno fiber: *no fiber: no EOL: flag: no action open EOL mode ACC & openxlink set flag CD restore Fiber: *fiber: EOL: flag: no action close EOLmode RIC & close xlink clear flag EF no fiber: *no fiber: no action openxlink EF restore fiber: *fiber: no action close xlink

FIG. 13 illustrates a simple analog audio Class B configuration with onemaster control panel 204 and three transponders 222. The components aresimilar to those shown in FIG. 12 and corresponding reference numbersare used where possible. Note that unlike the system of FIG. 12, thesystem of FIG. 13 has no return link from transponder 222C back to thealarm control panel 204, and thus there is no x-link between fibermodems A and F.

Table 5 below provides fault detection and recovery data for FIG. 13.Although Table 5 is similar to Table 4, rows should be read from rightto left. That is, actions begin with the right-most entry in a row(designated with an asterisk “*”) and propagate to the left.

TABLE 5 Class B analog audio fault detection and recovery ConfigurationModem A Modem B Modem E Modem F Fault head end RIC generic AAC genericRIC generic AAC 1 No detection or recover; controller handles faults. 2flag: *no EOL: open EOL set flag 2 restore flag: *EOL: close EOL clearflag 3 flag: no EOL: flag: *no EOL: open EOL set flag open EOL set flag3 restore flag: EOL: flag: *EOL: close EOL clear flag close EOL clearflag AB *no fiber: no fiber: open EOL no action AB restore *fiber:fiber: close EOL no action EF flag: no EOL: *no fiber: no fiber: openEOL set flag open EOL no action EF restore flag: EOL: *fiber: no fiber:close EOL clear flag close EOL no action

Table 6 below lists the conditions that are implemented in the FPGAlogic to accomplish the responses that are shown in the two analogconfiguration tables, i.e., Tables 4 and 5, above. The analog flagremains in its fault state until a fault clears. Modems do not latch thestate of the flag or their mode.

TABLE 6 Analog control output functional summary Mode If default = AAC,follow flag for mode switch. 1 = controller If default = RIC, follow EOLdetect for mode switch. Lock RIC if not generic or fiber fault. Lock indefault if class B. Any reset restores to default. X-link control Openif flag or fiber fault. 1 = open Lock closed if not tail end or class B.Any reset restores to default. Analog flag If default = AAC & class B,flag if no EOL. 1 = fault Lock if default—AAC and class A. If default =RIC & class A, flag if no EOL. Lock if default = RIC and class B. ClassA LED On if mode is not equal to default. 0 = on On if class B & no EOLor flag. On if fiber fault. EOLR/DC If default = AAC & class A, open ifflag or fiber fault. 1 = DC Lock off if default = AAC and class B. Ifdefault = RIC and class B, open if flag or fiber fault. If default = RICand class A, lock no EOL. Any reset restores to default.

FIG. 14, along with Tables 7 and 8 below, illustrates the methods offault detection, configuration control and recovery used for RUI routingcontrol. FIG. 14 shows a simple configuration, similar to that of FIG.12 and using corresponding reference numbers where possible, withindividual faults indicated by the small numbered or lettered boxes.Tables 7 and 8 show the method of detection of the fault and the processof entering degraded operation. These tables also show the method ofrecovery from degraded mode. Note that the term “14s flag” refers to thetail end modem flagging its mate every 14 seconds while in degraded modeto test for restoration of the fault. If the fault is still present whenthe flag is received, the modem returns to normal mode, and thenimmediately falls back into degraded mode.

The master control panel 204 includes an RUI card or similar controller212. Each of the transponders has a transponder interface card (TIC)215.

The rows of Table 7 corresponding to faults detected at 2, 3, AB, CD andEF are to be read from left to right, while the corresponding restoreoperations are to be read from right to left. First actions are markedwith an asterisk “*”.

TABLE 7 Class A RUI fault detection and recovery Configuration Modem AModem F head end Modem B Modem C Modem D Modem E tail end Fault TICgeneric RUI generic TIC generic RUI generic TIC TIC 1 Class A circuitsisolate fault; no modem mode changes. 2 *no DC: flag: no DC: flag: modeRUI & mode TIC mode RUI & open xlink flag flag 2 restore flag: no DC:flag: *14s flag: mode TIC mode mode TIC close RUI & flag xlink & flag 3*no DC: flag: mode RUI & open xlink flag 3 restore flag: *14s flag: modeTIC close xlink & flag 4 Class A circuits isolate fault; no modem modechanges 5 Class A circuits isolate fault; no modem mode changes AB nofiber: *no fiber: no DC: flag: no DC: flag: no action mode TIC mode RUI& mode TIC mode RUI & open xlink flag flag AB restore fiber: no DC:flag: no DC: flag: *14s flag: no action mode RUI mode TIC mode mode TICclose RUI & flag xlink & flag CD no fiber: *no fiber: no DC: flag: noaction mode TIC mode RUI & open xlink flag CD restore no fiber: no DC:flag: *14s flag: no change mode mode TIC close RUI & flag xlink & flagEF no fiber: *no fiber: no change open xlink EF restore fiber: fiber: nochange close xlink

Note that for class B operation, no mode changes are performed. Allmodems remain in their respective default modes.

It should be understood that although the components of FIGS. 12 and 14,such as an analog audio controller 210 (FIG. 12) and an RUI card 215C(FIG. 14) may both be present simultaneously, they are not showntogether for simplicity.

Table 8 below lists the conditions that are implemented in the FPGAlogic to accomplish the responses that are shown in Table 7. The RUIflag read may be triggered, for example, on a rising edge. Moderns latchtheir state. The 14 s timer is a 14-second timer that runs continuouslyin the tail-end modem. The tail-end modem closes its x-link and flagsits mate every time the timer rolls over, i.e., every 14 seconds, torecover from degraded mode. (If the x-link is already closed, then itremains closed.) If the mating modern is already in normal mode, thennothing happens.

TABLE 8 RUI control output functional summary Mode If default = RUI:switch if fiber fault or flag/ 1 = RUI recover if no DC. If default =TIC: switch if no DC/recover per flag. Lock TIC if RUI disabled or notgeneric. Lock default if class B. Any reset restores to default. X-linkcontrol Open if flag or fiber fault/recover when 14 s flag 1 = opentimer occurs. Lock if RUI disabled or class B or not tail end. Any resetrestores to default. RUI flag If current mode = TIC: flag if no DC. 1 =statchg Flag if tail and 14 s timer occurs. Class A LED On if currentmode is not equal to default mode OR 0 = on x-link open. Lock off if RUIdisabled or Class B. On if fiber fault.

Further Embodiments

FIG. 15 is a simplified schematic that covers several differentembodiments of a fire alarm network fiber optic multiplex modem 401. Themodem 401 comprises several local interfaces 403 including, but notlimited to, at least one of: a peer-to-peer protocol control panelcommunications interface; a master-to-slave protocol controlpanel/transponder communications interface; a digital audio interface;an analog audio interface; and a fire fighter phone interface. Althoughfive local interfaces 403 are shown, it would be understood by oneskilled in the art that any number of local interfaces may be present ina particular implementation.

A combiner/decombiner 405 combines data received at the local interfacesinto an outgoing optical combined signal. The combiner/decombiner 405also separates an incoming optical combined signal into its constituentdata streams and forwards each of the separate data streams to acorresponding local interface. The outgoing optical combined signal istransmitted, and the incoming optical combined signal is received, overa single optical fiber 409 through a fiber optic interface 407. Inparticular, the combiner/decombiner 405 can use, among other techniques,dense-wavelength-division-multiplexing and/or time-divisionmultiplexing, as described below.

Dense-Wavelength-Division-Multiplexing

An alternative embodiment of the invention usesdense-wavelength-division-multiplexing (DWDM) to multiplex the localinputs in the optical domain rather than in the time domain. Thereceived data at each local input forms a distinct output stream, and adifferent wavelength is dedicated for each output stream. The variousoutput streams are then transmitted concurrently over the fiber, eachwith its own dedicated wavelength channel. In addition, the modemconcurrently receives over the fiber multiple input streams, each atunique wavelength. A DWDM implementation would greatly reduce the logicrequired, but would significantly increase the cost of the opticalcomponents.

Alternatively, data from one or more subsets of the local inputs can becombined or multiplexed into distinct input streams using fiber framessimilar to that described previously. Each of these distinct inputstreams may be assigned a unique wavelength for transmission on thefiber. Incoming data from the fiber may be treated similarly. Forexample, digital audio and fire fighter phone audio can be combined intoa single output stream and transmitted at one wavelength, while networkcommunications data forms a second output stream, transmitted at adifferent wavelength.

Time Division Multiplexing

In yet another embodiment of the present invention, time divisionmultiplexing (TDM), already used as discussed previously to multiplexlocal inputs into the optical stream, is also used to multiplex the twodata directions (outgoing and incoming) on the optical interface in lieuof wavelength division multiplexing (WDM). In such an implementation,one modem may be designated as the master, and one as the slave. Themaster controls the communications channel, while the slave follows themaster. The input data may be compressed to a format similar to thatpreviously described, although further compression in the time domainmay be necessary so that a frame can be transmitted in half (at most) ofthe normal duration, since the other half of the normal duration isrequired for the slave to transmit its frame. The master transmits itsdata to the slave. The slave may immediately follow by transmitting itsown data back to the master. In such an implementation, a singlewavelength can be used for both directions. While this could complicatethe logical implementation, it may significantly reduce the cost of theoptical components.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A fire alarm network fiber optic multiplex modem comprising: at leastone interface configured to generate a data stream; a fiber opticinterface; a fiber optic modem configured to transmit the data stream tothe fiber optic interface and configured to receive an incoming datastream via the fiber optic interface; and a fault detector which, upondetection of a fault in a communications path, is configured to signalan indication of the fault to a second fiber optic multiplex modem viathe fiber optic interface.
 2. The fiber optic multiplex modem of claim1, wherein the fault detected comprises a failure to receive a validincoming data stream via the communication path.
 3. The fiber opticmultiplex modem of claim 1, wherein the fault detected comprises afailure to detect electrical continuity in the at least one localinterface.
 4. The fiber optic multiplex modem of claim 1, wherein thefault detector is configured to signal the indication of the fault tothe second fiber optic multiplex modem by sending a communication to thesecond fiber optic multiplex modem via the fiber optic interface.
 5. Thefiber optic multiplex modern of claim 4, wherein the communicationincludes a control bit indicative of the fault.
 6. The fiber opticmultiplex modem of claim 1, wherein the fault detector is configured tosignal the indication of the fault to the second fiber optic multiplexmodem by removing an end-of line resistor.
 7. A method for communicatingbetween nodes in a fire alarm network, the method comprising: generatingan outgoing data stream from data received from at least one localinterface; transmitting the outgoing data stream to a fiber opticinterface; receiving an incoming data stream via the fiber opticinterface; forwarding at least a part of the incoming data stream to theat least one local interface; detecting a fault in a communicationspath; and signaling an indication of the fault to a second fiber opticmultiplex modem via the fiber optic interface.
 8. The method of claim 7,wherein the fault detected comprises a failure to detect electricalcontinuity in the at least one local interface.
 9. The method of claim7, wherein the fault detected comprises a failure to detect electricalcontinuity in the at least one local interface.
 10. The method of claim7, wherein signaling an indication of the fault to a second fiber opticmultiplex modem via the fiber optic interface comprises sending acommunication to the second fiber optic multiplex modem via the fiberoptic interface.
 11. The method of claim 10, wherein the communicationincludes a control bit indicative of the fault.
 12. The method of claim7, wherein signaling an indication of the fault to a second fiber opticmultiplex modem via the fiber optic interface comprises removing anend-of line resistor.
 13. A fire alarm network fiber optic multiplexmodem comprising: at least one local interface; a fiber optic interface;a fiber optic modem configured to communicate with a second fiber opticmodem via the fiber optic interface and configured to communicate withat least one separate device via the local interface; and a faultdetector configured to determine an indication of a fault in acommunications path and configured to, in response to determining theindication of the fault, modify at least one aspect of an electricalconnection to the local interface.
 14. The fire alarm network fiberoptic multiplex modem of claim 13, wherein the fault detector isconfigured to determine the indication of the fault by receiving acommunication from the second fiber optic modem.
 15. The fire alarmnetwork fiber optic multiplex modem of claim 13, wherein the faultdetector is configured to determine the indication of the faultresponsive to a failure to receive a valid incoming data stream via thecommunications path.
 16. The fire alarm network fiber optic multiplexmodem of claim 13, wherein the fault detector, upon determination of thefault in the communications path that extends from the fiber opticinterface, is configured to disconnect the electrical connection to theat least one interface, thereby creating an open-circuit in anelectrical path with the at least one separate device.
 17. The firealarm network fiber optic multiplex modem of claim 13, wherein the faultdetector is configured to modify at least one aspect of the electricalconnection based on a type of the fault.
 18. The fire alarm networkfiber optic multiplex modem of claim 17, wherein the type of fault is ashort-circuit; and wherein the fault detector is configured to simulatea short-circuit in an electrical path with the at least one separatedevice.
 19. The fire alarm network fiber optic multiplex modem of claim13, wherein the communication path comprises a cross-link.